Biomass refining (or biorefining) is becoming more prevalent in industry. Cellulose fibers and sugars, hemicellulose sugars, lignin, syngas, and derivatives of these intermediates are being used by many companies for chemical and fuel production. Indeed, we now are observing the commercialization of integrated biorefineries that are capable of processing incoming biomass much the same as petroleum refineries now process crude oil. Underutilized lignocellulosic biomass feedstocks have the potential to be much cheaper than petroleum, on a carbon basis, as well as much better from an environmental life-cycle standpoint.
Lignocellulosic biomass is the most abundant renewable material on the planet and has long been recognized as a potential feedstock for producing chemicals, fuels, and materials. Lignocellulosic biomass normally comprises primarily cellulose, hemicellulose, and lignin. Cellulose and hemicellulose are natural polymers of sugars, and lignin is an aromatic/aliphatic hydrocarbon polymer reinforcing the entire biomass network. Some forms of biomass (e.g., recycled materials) do not contain hemicellulose.
There are many reasons why it would be beneficial to process biomass in a way that effectively separates the major fractions (cellulose, hemicellulose, and lignin) from each other. Cellulose from biomass can be used in industrial cellulose applications directly, such as to make paper or other pulp-derived products. The cellulose can also be subjected to further processing to either modify the cellulose in some way or convert it into glucose. Hemicellulose sugars can be fermented to a variety of products, such as ethanol, or converted to other chemicals. Lignin from biomass has value as a solid fuel and also as an energy feedstock to produce liquid fuels, synthesis gas, or hydrogen; and as an intermediate to make a variety of polymeric compounds. Additionally, minor components such as proteins or rare sugars can be extracted and purified for specialty applications.
In light of this objective, a major shortcoming of previous process technologies is that one or two of the major components can be economically recovered in high yields, but not all three. Either the third component is sacrificially degraded in an effort to produce the other two components, or incomplete fractionation is accomplished. An important example is traditional biomass pulping (to produce paper and related goods). Cellulose is recovered in high yields, but lignin is primarily consumed by oxidation and hemicellulose sugars are mostly degraded. Approximately half of the starting biomass is essentially wasted in this manufacturing process. State-of-the-art biomass-pretreatment approaches typically can produce high yields of hemicellulose sugars but suffer from moderate cellulose and lignin yields.
There are several possible pathways to convert biomass into intermediates. One thermochemical pathway converts the feedstock into syngas (CO and H2) through gasification or partial oxidation. Another thermochemical pathway converts biomass into liquid bio-oils through pyrolysis and separation. These are both high-temperature processes that intentionally destroy sugars in biomass.
Sugars (e.g., glucose and xylose) are desirable platform molecules because they can be fermented to a wide variety of fuels and chemicals, used to grow organisms or produce enzymes, converted catalytically to chemicals, or recovered and sold to the market. To recover sugars from biomass, the cellulose and/or the hemicellulose in the biomass must be hydrolyzed into sugars. This is a difficult task because lignin and hemicelluloses are bound to each other by covalent bonds, and the three components are arranged inside the fiber wall in a complex manner. This recalcitrance explains the natural resistance of woody biomass to decomposition, and explains the difficulty to convert biomass to sugars at high yields.
Fractionation of biomass into its principle components (cellulose, hemicellulose, and lignin) has several advantages. Fractionation of lignocellulosics leads to release of cellulosic fibers and opens the cell wall structure by dissolution of lignin and hemicellulose between the cellulose microfibrils. The fibers become more accessible for hydrolysis by enzymes. When the sugars in lignocellulosics are used as feedstock for fermentation, the process to open up the cell wall structure is often called “pretreatment.” Pretreatment can significantly impact the production cost of lignocellulosic ethanol.
One of the most challenging technical obstacles for cellulose has been its recalcitrance towards hydrolysis for glucose production. Because of the high quantity of enzymes typically required, the enzyme cost can be a tremendous burden on the overall cost to turn cellulose into glucose for fermentation. Cellulose can be made to be reactive by subjecting biomass to severe chemistry, but that would jeopardize not only its integrity for other potential uses but also the yields of hemicellulose and lignin.
Many types of pretreatment have been studied. A common chemical pretreatment process employs a dilute acid, usually sulfuric acid, to hydrolyze and extract hemicellulose sugars and some lignin. A common physical pretreatment process employs steam explosion to mechanically disrupt the cellulose fibers and promote some separation of hemicellulose and lignin. Combinations of chemical and physical pretreatments are possible, such as acid pretreatment coupled with mechanical refining. It is difficult to avoid degradation of sugars. In some cases, severe pretreatments (i.e., high temperature and/or low pH) intentionally dehydrate sugars to furfural, levulinic acid, and related chemicals. Also, in common acidic pretreatment approaches, lignin handling is very problematic because acid-condensed lignin precipitates and forms deposits on surfaces throughout the process.
One type of pretreatment that can overcome many of these disadvantages is called “organosolv” pretreatment. Organosolv refers to the presence of an organic solvent for lignin, which allows the lignin to remain soluble for better lignin handling. Traditionally, organosolv pretreatment or pulping has employed ethanol-water solutions to extract most of the lignin but leave much of the hemicellulose attached to the cellulose. For some market pulps, it is acceptable or desirable to have high hemicellulose content in the pulp. When high sugar yields are desired, however, there is a problem. Traditional ethanol/water pulping cannot give high yields of hemicellulose sugars because the timescale for sufficient hydrolysis of hemicellulose to monomers causes soluble-lignin polymerization and then precipitation back onto cellulose, which negatively impacts both pulp quality as well as cellulose enzymatic digestibility.
An acid catalyst can be introduced into organosolv pretreatment to attempt to hydrolyze hemicellulose into monomers while still obtaining the solvent benefit. Conventional organosolv wisdom dictates that high delignification can be achieved, but that a substantial fraction of hemicellulose must be left in the solids because any catalyst added to hydrolyze the hemicellulose will necessarily degrade the sugars (e.g., to furfural) during extraction of residual lignin.
Contrary to the conventional wisdom, it has been found that fractionation with a solution of ethanol (or another solvent for lignin), water, and sulfur dioxide (SO2) can simultaneously achieve several important objectives. The fractionation can be achieved at modest temperatures (e.g., 120-160° C.). The SO2 can be easily recovered and reused. This process is able to effectively fractionation many biomass species, including softwoods, hardwoods, agricultural residues, and waste biomass. The SO2 hydrolyzes the hemicelluloses and reduces or eliminates troublesome lignin-based precipitates. The presence of ethanol leads to rapid impregnation of the biomass, so that neither a separate impregnation stage nor size reduction smaller than wood chips are needed, thereby avoiding electricity-consuming sizing operations. The dissolved hemicelluloses are neither dehydrated nor oxidized (Iakovlev, “SO2-ethanol-water fractionation of lignocellulosics,” Ph.D. Thesis, Aalto Univ., Espoo, Finland, 2011). Cellulose is fully retained in the solid phase and can subsequently be hydrolyzed to glucose. The mixture of hemicellulose monomer sugars and cellulose-derived glucose may be used for production of biofuels and chemicals.
Commercial sulfite pulping has been practiced since 1874. The focus of sulfite pulping is the preservation of cellulose. In an effort to do that, industrial variants of sulfite pulping take 6-10 hours to dissolve hemicelluloses and lignin, producing a low yield of fermentable sugars. Stronger acidic cooking conditions that hydrolyze the hemicellulose to produce a high yield of fermentable sugars also hydrolyze the cellulose, and therefore the cellulose is not preserved.
The dominant pulping process today is the Kraft process. Kraft pulping does not fractionate lignocellulosic material into its primary components. Instead, hemicellulose is degraded in a strong solution of sodium hydroxide with or without sodium sulfide. The cellulose pulp produced by the Kraft process is high quality, essentially at the expense of both hemicellulose and lignin.
Sulfite pulping produces spent cooking liquor termed sulfite liquor. Fermentation of sulfite liquor to hemicellulosic ethanol has been practiced primarily to reduce the environmental impact of the discharges from sulfite mills since 1909. However, ethanol yields do not exceed one-third of the original hemicellulose component. Ethanol yield is low due to the incomplete hydrolysis of the hemicelluloses to fermentable sugars and further compounded by sulfite pulping side products, such as furfural, methanol, acetic acid, and others fermentation inhibitors.
Solvent cooking chemicals have been attempted as an alternative to Kraft or sulfite pulping. The original solvent process is described in U.S. Pat. No. 1,856,567 by Kleinert et al. Groombridge et al. in U.S. Pat. No. 2,060,068 showed that an aqueous solvent with sulfur dioxide is a potent delignifying system to produce cellulose from lignocellulosic material. Three demonstration facilities for ethanol-water (Alcell), alkaline sulfite with anthraquinone and methanol (ASAM), and ethanol-water-sodium hydroxide (Organocell) were operated briefly in the 1990s.
In view of the state of the art, what is desired is to efficiently fractionate any lignocellulosic-based biomass (including, in particular, softwoods) into its primary components so that each can be used in potentially distinct processes. While not all commercial products require pure forms of cellulose, hemicellulose, or lignin, a platform biorefinery technology that enables processing flexibility in downstream optimization of product mix, is particularly desirable. An especially flexible fractionation technique would not only separate most of the hemicellulose and lignin from the cellulose, but also render the cellulose highly reactive to cellulase enzymes for the manufacture of fermentable glucose.
The AVAP® fractionation process developed by American Process, Inc. and its affiliates is able to economically accomplish these objectives. Improvements are still desired in the areas of lignin separation and recovery.
One of the biggest and well-known challenges in many biorefineries is dealing with lignin. Lignin is a major component of biomass. It is typically between 15-35 wt % (dry basis) of the biomass material. Lignin has good fuel value, similar to some types of coal.
Lignin is a natural polymer and is an essential part of wood and other forms of cellulosic biomass, including agricultural crop residues such as sugarcane bagasse. Lignin performs multiple functions that are essential to the life of the plant, including transport of nutrition and durability of the biomass. Lignin imparts rigidity to the cell walls and acts as a binder, creating a flexible composite cellulose-hemicellulose-lignin material that is outstandingly resistant to impact, compression, and bending.
After polysaccharides (polymers of sugar), lignin is the most abundant organic polymer in the plant world. Lignin is a very complex natural polymer with many random couplings, and therefore lignin has no exact chemical structure. The molecular structure of lignin consists primarily of carbon ring structures (benzene rings with methoxyl, hydroxyl, and propyl groups.
Various processes can be used to remove and isolate lignin from biomass. Each process, however, produces material of different composition and properties. Generally there are four important factors to take into account when working with lignin:
1. Source of the lignin.
2. Method used to remove lignin from the biomass.
3. Method(s) used to purify the lignin.
4. Nature of the chemical modification of the lignin after isolation.
These factors influence the properties of the lignin. Important properties of lignin formulations include molecular weight, chemical composition, and the type and distribution of chemical functional groups.
Separation and recovery of lignin is quite difficult. It is possible to break the lignin-cellulose-hemicellulose matrix and recover the lignin through a variety of treatments on the lignocellulosic material. However, known lignin recovery methods generally have one or more important commercial-scale limitations. Lignin purification from biomass is a classic chemical-engineering problem with complex chemistries and transport phenomena, criticality of reactor design and scale-up, serious analytical challenges, and many practical issues arising from lignin's propensity to stick to equipment and piping.
Lignin can be difficult to process in biorefineries because it has a tendency to deposit on solid surfaces and cause plugging. Although lignin handling has always been known to be a challenge, there remains a need in the art for ways to either avoid lignin precipitation or to deal with it after it occurs. Other difficulties are caused by downstream fermentation inhibition caused by lignin, as well as lignin fragments and derivatives (e.g., phenolics, acids, and other compounds).
Lignin separations challenges appear to be particularly troubling problem for acidic pretreatments of biomass or biomass-derived liquors. For example, in van Heiningen et al., “Which fractionation process can overcome the techno-economic hurdles of a lignocellulosic biorefinery,” Proceedings of the AIChE Annual Meeting, Minneapolis, Minn. (2011), it is cautioned that “an operating problem which has mostly been overlooked for acidic pretreatment is formation and precipitation of sticky lignin on reactor walls and piping.” The lack of R&D attention to this problem is stated to be that it only “becomes apparent in continuous larger scale operation after one to two week operation.”
In view of the aforementioned needs in the art, improvements are clearly needed to avoid, or deal with, lignin precipitation during acidic hydrolysis of biomass and/or biomass hydrolysates (such as hemicellulose-containing liquid extracts).